1. Field of the Invention
The present invention relates generally to wide-range tunable semiconductor lasers and particularly to sampled-grating distributed Bragg reflector (SGDBR) lasers.
2. Description of the Related Art
Diode lasers are being used in such applications as optical communications, sensors and computer systems. In such applications, it is very useful to employ lasers that can be easily adjusted to output frequencies across a wide wavelength range. A diode laser which can be operated at selectably variable frequencies covering a wide wavelength range is an invaluable tool. The number of separate channels that can utilize a given wavelength range is exceedingly limited without such a laser. Accordingly, the number of individual communications paths that can exist simultaneously in a system employing such range-limited lasers is similarly very limited. Thus, while diode lasers have provided solutions to many problems in communications, sensors, and computer system designs, they have not fulfilled their potential based on the available bandwidth afforded by light-based systems. It is important that the number of channels be increased in order for optical systems to be realized for many future applications.
For a variety of applications, it is necessary to have tunable single-frequency diode lasers which can select any of a wide range of wavelengths. Such applications include sources and local oscillators in coherent lightwave communications systems, sources for other multi-channel lightwave communication systems, and sources for use in frequency modulated sensor systems. Continuous tunability is usually needed over some range of wavelengths. Continuous tuning is important for wavelength locking or stabilization with respect to some other reference, and it is desirable in certain frequency shift keying modulation schemes.
Sampled-grating distributed Bragg reflector (SGDBR) lasers obtain many of these desirable features through the use of two sampled gratings bounding a gain and phase section of a semiconductor. The basic function and structure of SGDBR lasers is detailed in U.S. Pat. No. 4,896,325, issued Jan. 23, 1990, to Larry A. Coldren, entitled xe2x80x9cMULTI-SECTION TUNABLE LASER WITH DIFFERING MULTI-ELEMENT MIRRORSxe2x80x9d, which patent is incorporated by reference herein. However, designing an optimized SGDBR laser for a given application can be a complex, time consuming, iterative process. There is a need in the art for methods and devices that facilitate simpler, more manufacturable designing of SGDBR lasers based on application-specific performance criteria.
To address the issues described above, the present invention discloses a tunable laser comprised of a gain section for creating a light beam by spontaneous and stimulated emission over a bandwidth, a phase section for controlling the light beam around a center frequency of the bandwidth, a cavity for guiding and reflecting the light beam, a front mirror bounding an end of the cavity, and a back mirror bounding an opposite end of the cavity. The back mirror has a xcexaeffB approximately equal to xcex1Tune, where xcexaeffB is an effective coupling constant and xcex1Tune is an amount of propagation loss anticipated for an amount of peak tuning required, and a length of the back mirror is made to produce greater than approximately 70% reflectivity (at a minumum, to approximately 80% reflectivity at a maximum) in combination with a front mirror that is intended to produce less than approximately 25% reflectivity.